The present invention relates to vacuum gauges and more particularly to ionization gauges for use over a wide pressure range.
Ionization gauges are, in general, known. Such gauges typically comprise a source of electrons (cathode), an accelerating electrode (anode) to provide energetic electrons, and a collecting electrode (collector) to collect the ions formed by electrons impacting on gas molecules or atoms within the gauge. The number of positive ions formed within the gauge (in a gas susceptible to ionization by electron impact) is directly proportional to the molecular concentration of gas within the gauge. However, the production of undesirable extraneous currents in the gauge, which are independent of gas pressure, tend to present a practical barrier to measurement of ultra-high vacuums.
The undesirable extraneous currents principally result from a so-called X-ray effect. Bombardment of the anode by electrons produces soft X-rays. The soft X-rays impinge on the collector, thereby producing a photo-electron current which adds to the ion current in the collector. The photo-electron current and the ion current are not distinguishable from one another in the ion current measuring circuit. Thus, the photo-electron current establishes a lowest practical limit beyond which meaningful ion current measurement cannot be had.
In general, vacuum gauges which have successfully reduced the X-ray effect by several orders of magnitude and permitted pressure measurement to at least 10.sup.-10 Torr and with special precautions to still lower pressures are known. Such a gauge, commonly referred to as the "Bayard-alpert (BA) gauge," is disclosed in U.S. Pat. No. 2,605,431 issued July 29, 1952 to Bayard. See also U.S. Pat. No. 4,307,323 issued on Dec. 22, 1981 to Bills et al. The BA ionization gauge is widely used. However, because low pressure gauge calibration is a very expensive and time-consuming procedure, most BA transducers are used as manufactured, and are typically not subjected to calibration before use. Thus, it is highly desirable that the gauge sensitivity be highly reproducible and stable with use.
Unfortunately, the sensitivity of commercially available BA gauges tends to be neither reproducible, nor stable. It has been found that typical commercially available BA gauges exhibit substantial differences in sensitivity from the nominal value of sensitivity specified by the manufacturer. See K. E. McCulloh and C. R. Tilford, J. Vac. Sci. Technol. 18 994 (1981). It has also been found that sensitivity in the same gauge assembly tends to differ when operation is switched from one filament to another. Further, it has been noted that the sensitivity of typical BA gauges tends to drift by, for example, as much as -1.4% per 100 operating hours when kept at vacuum. Moreover, changes in sensitivity (of up to 25%) occur when the gauge is briefly exposed to the atmosphere and then operated in vacuum. See K. F. Poulter and C. M. Sutton, Vacuum 31 147 (1981).
Ionization gauges have been made which exhibit sensitivities which are reproducible and stable to better than .+-.2% over an 18-month period. However, these transducers are elaborate, complex and costly devices not suited for general use and are incapable of measuring very low pressures. See K. F. Poulter et al, J. Vac. Sci. Technol. 17 679 (1980).
It has been determined that changes in a number of gauge parameters in particular tend to produce variations (from gauge to gauge and within the same gauge from use to use) in ion current for a given constant pressure and constant emission current: (a) the electron current effective to produce ions; (b) the ionizing energy; (c) the total electron path length; and/or (d) the ion collection efficiency.
The electric field in the prior art gauges varies from place to place in the gauge. Accordingly, the ionizing energy that an electron acquires depends both upon the particular trajectory of the electron and the instantaneous position of the electron along the trajectory. However, it is well-known that electrons emitted from different portions of the cathode follow greatly different trajectories in a BA gauge. Electron paths vary greatly depending on where on the cathode and in which direction the electron is emitted. See, for example, L. G. Pittaway, J. Phys. D. Appl. Phys. 3 1113 (1970).
Free-standing electrodes are commonly used in ionization gauges. Examples are described in U.S. Pat. Nos. 3,742,343 issued June 26, 1978 to Pittaway, and 3,839,655 issued Oct. 1, 1974 to Helgeland et al, and in P. A. Redhead, J. Vac. Sci. Technol., 3 173 (1966). Such electrode structures, however, are prone to creep and sag with use. It has been observed that seemingly negligible variations in electrode geometry in the prior art gauges, due to, for example, small manufacturing tolerances, or creep and sag of the electrode, produce large changes in number of electrons transmitted and drastically affect electron trajectories (and thus total electron path length) in the ion collection volume.
Since, in prior art devices, the trajectory of an electron is dependent upon point of origin on the cathode, if the pattern of emitted electrons from the cathode varies, the total electron path length and the ionizing effectiveness in the gauge will vary. Unfortunately, as is well-known, the emission pattern from a hot cathode is drastically affected by localized changes in the work function of the cathode surface due to contamination, by changes in the emissivity of the emitting surface, and by changes in the cathode temperature. For example, thoria coated refractory metal cathodes are commonly used in ionization gauges. Cracking and spalling of the coating from the refractory metal base can lead to relatively large localized temperature changes resulting in large changes in the emission pattern. Also, crystal formation in pure refractory metal cathodes can cause localized changes in work function which can drastically affect the emission pattern.
Attempts have been made to control the divergence of the emitted electron stream from the cathode to anode. For example, a special electrode has been placed behind the cathode for this purpose. Such a gauge is described in U.S. Pat. No. 3,743,876 to P. A. Redhead on July 3, 1973.
Additional reasons for the non-reproducible and unstable prior art gauge sensitivities have been noted. It is well-known in the art of electron tube devices that electrons are preferentially focused on grid wires held at a positive potential with respect to the cathode. Thus, the fraction of electrons transmitted through the grid in a BA gauge is substantially less than would be estimated from the geometrical transparency of the grid. Empirical observations show less than 50% of the incident electrons are transmitted through a grid with 85% geometrical transparency.
In addition, the ion collection volume in the prior art gauges tends to be neither reproducible nor stable. The ion collection volume is the volume, within the gauge anode within which a positive ion with zero initial velocity is attracted to and collected by the ion collector. In prior art gauges utilizing an open grid, such as a BA gauge, the electric field leaks through the open grid. Accordingly, ions formed near the grid experience an electric force urging them out of the grid volume, rather than an electric force urging them toward the ion collector. This leakage of the electric field into the grid volume considerably reduces the volume from which positive ions are collected by the ion collector. If the grid electrode in prior art gauges was entirely reproducible and remain stable with use, the decreased ion collection volume would merely result in decreased sensitivity. However, because the grids in the prior art gauges are purposely flimsy, the grids and, therefore, the gauge sensitivity, are neither reproducible nor stable.
Thus, prior art gauges tend not to have reproducible and stable gauge sensitivities. The emission pattern varies from cathode to cathode, and varies even in respect to an individual cathode with extended use and with exposure to air or oxygen. Thus, the electron trajectories change, producing changing path length and varying sensitivity. The use of grids and asymmetrical cathodes causes the gauges to be enormously sensitive to small variations in uncontrollable parameters. Emission patterns are essentially non-controllable, and manufacturing tolerances, creep and sag in the prior art gauges cannot be reduced economically.